Substantial progress in understanding the responses to tumor-derived growth factor-.beta. (TGF-.beta.) and related ligands has been made in the last five years (Derynck and Fang, 1997; Hoodless and Wrana, 1998; Kretzschmar and Massague, 1998). The receptors for these ligands have been cloned and shown to be serine/threonine kinases which are activated by binding to ligand. The major substrates for these kinases, besides the receptors themselves, appear to be Smad proteins. The founding member of the Smad family is the product of the Drosophila gene Mad, identified by its requirement in signaling by the TGF-.beta. family member Dpp (Sekelsky et al., 1995). Nine homologs of Mad have since been identified in vertebrate cells and shown to transduce or inhibit signals from specific TGF-.beta. like ligands (Heldin et al., 1997; Derynck and Fang, 1997; Hoodless and Wrana, 1998; Kretzschmar and Massague, 1998).
The phosphorylation of Smad 1, Smad2, and Smad3 stimulates their interaction with Smad4 and the transport of the resulting heteromeric complex to the nucleus (Kretzschmar et al., 1997; Lagna et al., 1996; Liu, 1997; Macias-Silva et al., 1996; Nakao et al., 1997; Nakao et al., 1997; Souchelnytskyi et al., 1997). Once in the nucleus, the Smad complex transcriptionally activates specific target genes through activation domains present at the carboxyl termini of these proteins (Liu et al., 1996). Two ways in which Smad activation could lead to transcriptional activation have been identified. First, it has been shown that human Smad3 and Smad4, but not Smad2, can bind to specific DNA sequences and activate transcription of adjacent reporters (Zawel et al., 1998). A similar sequence-specific activity is present in Drosophila Mad (Kim et al., 1997). Second, Smad2 has been shown to bind to the Xenopus forkhead activin signal transducer protein FAST-1 (xFAST-1) and to participate in a complex exhibiting sequence specific binding activity attributable to the xFAST-1 component (Chen et al., 1996; Chen et al., 1997; Liu, 1997). Although Smad4 does not directly bind to xFAST-1, Smad4 is recruited to the xFAST-1/Smad2 complex by Smad2 (Chen et al., 1997; Liu, 1997).
TGF-.beta.-like responses are remarkably widespread in eukaryotes, and are important not only in development but also in cancer (Fynan and Reiss, 1993; Hartsough and Mulder, 1997). Further progress in understanding the varied developmental and oncogenic ramifications of these pathways in mammalian cells depends on knowledge of the relevant mammalian genes. Thus, there is a need in the art for the identification, isolation, purification, and analysis of mammalian and human genes which mediate physiological and pathological responses to TGF-.beta. and related ligands.